New estimates on global warming

The global climate could warm by 2.5 degrees F. by the end of the century, even if countries undertake stringent efforts to curb greenhouse-gas emissions, according to a new study from a team of climate scientists in the United States, China, Japan, and five European nations.

This figure is more than twice as high as the level cited in the most recent report by the Intergovernmental Panel on Climate Change (IPCC), when it estimated how much global warming could occur even if nations froze emission at year-2000 levels.

The new estimate is a byproduct of a study the team conducted to gauge the effect of different emission-reduction efforts on Earth’s climate by 2100. The greenhouse-gas emissions scenarios that the IPCC typically uses don’t include the effects of government policies that directly tackle climate change, the researchers say. Instead, the IPCC looks at policies on population growth, the economy, and other slightly tangential policies that affect global warming. But, the team adds, a lot of progress has been made developing emissions scenarios that take climate-change-specific policies into account.

Using a selection of these new scenarios, the team calculates that countries could hold global average temperature increases to between 0.9 degrees F. and 7.9 degrees F. above 1990 levels by 2100, depending on how aggressive emission-reduction policies are. Those figures are 0.5 to 6.1 degrees F. lower than the increases one might expect if no mitigation policies are adopted.

The results are based in part on several simplifying assumptions, the team acknowledges. For instance, the models assume that policies are adopted globally, economy-wide, and without regard to their chance of actually being passed into law, nationally or internationally.Still, the team says, the results show that “ambitious efforts can significantly reduce global warming” and that the “residual warming” of 2.5 degrees underscores the need for efforts to not only slow climate change, but also to adapt to the changes that are unavoidable. The results appear in this week’s issue of the Proceedings of the National Academy of Sciences.